Issue #20: !17 (#16) made the x86-64-v3 baseline opt-in via RUSTFLAGS but
nothing set it, so release binaries silently fell back to the plain x86-64
baseline. Set RUSTFLAGS per matrix row on the two x86_64 entries only:
- linux x86_64-unknown-linux-gnu: -C target-cpu=x86-64-v3
- windows x86_64-pc-windows-msvc: -C target-cpu=x86-64-v3
-C link-arg=/STACK:8388608 (carries the 8 MB stack link-arg that the
repo config supplies for Windows, because RUSTFLAGS REPLACES config
rustflags rather than appending)
The aarch64 rows keep the config's link-args untouched (no RUSTFLAGS at
all), and macOS / android jobs plus .cargo/config.toml are unchanged.
Closes#20
